1. Field of the Invention
This invention relates to a device for feeding electric power to a roll-like revolving electrode in a resistance welding machine.
The term "revolving electrode", as used herein, denotes a type of electrode which rotates while moving in a predetermined direction. It should be understood therefore that the terms "revolving" as a qualifier and "revolve" as a verb are used in this sense in the present specification and the appended claims.
2. Description of the Prior Art
Resistance welding machines including a revolving electrode have been widely known for welding metallic can bodies to make cans for holding aerosols, beer, powdery coffee, etc. and 18-liter cans.
In resistance welding machines of this type, a can body is welded by passing a relatively large electrical current through an elongate electrode and a revolving electrode that moves with rotation while the can body is being pressed between these electrodes. It is necessary for this purpose to couple both the elongate electrode and the revolving electrode to a power supply, and to move at least one of them relative to the power supply. It is the general practice in the conventional welding machines of this type to couple the power supply electrically to the revolving electrode using a flexible feeder line.
The conventional resistance welding machines, however, have various problems to be solved as described below.
Firstly, the feeder line undergoes marked wear. In these machines, the revolving electrode reciprocates longitudinally over a stroke corresponding to the sum of the welding length and the length required for acceleration and deceleration, and also moves vertically. The reciprocal movement of the revolving electrode deforms the flexible feeder line. In particular, when it reciprocates at a high speed or over a long stroke, the feeder line is worn out within a short period of time and cannot withstand use for a long time.
Secondly, heat generation is vigorous since the cross-sectional area and shape of the feeder line are restricted. The feeder line must be deformed according to the reciprocating motion of the revolving electrode. Hence, the feeder line must be slender and thin in its cross section. This shape of the feeder line leads to an increase in the electric resistance part of the impedance of the feeder line, and consequently heat generation in the feeder line increases. It is generally difficult, moreover, to cool such a deformable feeder line forcibly by such a means as water cooling.
Thirdly, the value of the electric current varies. If the flexible feeder line is deformed, the impedance of a circuit including the feeder line, particularly its inductance part, varies. As a result, the value of the current used for welding considerably varies depending upon the deformed condition of the feeder line and therefore proper positioning of the revolving electrode, and proper welding is difficult to perform.
Fourthly, it is difficult to pass a large electrical current through the feeder line. The aforesaid machines require a relatively large current for welding. However, since it is necessary to increase the distance between the feeder line and constituent members of the circuit, the inductance part of the impedance of the circuit increases, and it is difficult to pass a large electric current.
Fifthly, there is an increase in the load of the rectilinear reciprocating motion of the revolving electrode. In the aforesaid machines, the revolving electrode for its driving mechanism is connected to the feeder line through a bearing. Hence, the revolving electrode must be moved against the force of the feeder line to resist deformation, and this results in an increase in the load of the rectilinear reciprocating motion of the revolving electrode. Particularly, when the revolving electrode reciprocates at a high speed, the weight of the feeder line (the force of inertia) cannot be ignored.